1. Field of the Invention
The present invention relates to a method for adjusting the overall luminosity of a bistable matrix screen displaying half-tones. It also relates to a display device that uses the method.
The invention can be applied to screens of the type having an internal memory. A screen with an internal memory is a screen whose cells, which form the pixels, preserve the "written" state or the "extinguished" state after the end of the signal activating the "written" state or the "extinguished" state as is the case notably with plasma panels and especially with alternating type plasma panels.
The screens to which the invention applies comprise elementary cells arranged in rows and columns in matrix form.
The use of display screens in a very wide variety of luminous environments may lead to the adjusting of their overall luminance as a function of the ambient luminosity in which they are used. In fact, it is recommended that the overall luminance of the screen should be comparable to that of the environment, otherwise unnecessary fatigue will be created for the user. The conditions of illumination around the screen may vary by a factor of about 1000 (from some tens of lux indoors with attenuated illumination to some tens of thousands of lux outdoors in sunlight).
The description shall be made in the case of a alternating type of plasma display panel. However, the invention can be applied to other types of bistable display panels, for example liquid-crystal display screens.
2. Description of the Prior Art
The working and structure of alternating-type plasma panels is well known. These panels, are for example, of the crossed-electrode type defining a cell as described in the French patent FR-2 417 848. The addressing of a given cell is achieved by the selection of two crossed electrodes to which appropriate voltages are applied at a given instant so that the difference in potential prompts a writing discharge or an erasure discharge between these electrodes.
A standard method of addressing consists of a row-by-row operation. In this case, all the cells of a given row simultaneously receive a command, by means of a semi-selective operation, for them to be erased or written on, for example to be erased, and this operation is followed by a selective operation during which at least one of the cells of the row is written on.
The semi-selective operation followed by the selective operation is accomplished, for each row, with a time lag from one row to the other corresponding to the duration of a row cycle.
Generally, the addressing by semi-selective operation and selective operation is done by a method in which addressing square-wave signals are overlaid on basic square-wave signals as explained, for example, in the patents FR-2 635 901 and FR-2 635 902.
These basic square-wave signals are applied simultaneously to all the cells for a period constituting an addressing stage and the addressing square-wave signals are overlaid on these basic square-wave signals only for the rows of cells addressed with, from one row to the other, the time lag corresponding to the duration of a row cycle T1; this means that the starting points of two consecutive addressing stages are separated by the duration of the row cycle.
Generally, in each row cycle, the addressing stage is followed by a sustaining stage during which the cells in the written state are activated, i.e. they produce light. Indeed, in this sustaining stage, sustaining signals are applied simultaneously to all the cells and prompt sustaining discharges that provide the essential part of the light emission perceived by an observer.
The sustaining signal is an alternating signal formed by voltage square waves that succeed one another with opposite polarities: each change in sign of the alternating signal (leading edges or trailing edges) generates a discharge in the gas or an emission of light in the cell or cells concerned. Thus, the quantity of light emitted by a cell in the illuminated state, namely the written state, is substantially proportional to the number of edges corresponding to polarity changes and, consequently, to the frequency of the sustaining signal.
It must be noted that in the addressing stage, as regards both recording and erasure, the basic square-wave signals have substantially one and the same amplitude as the sustaining signals and, consequently, they too may generate discharges comparable to the sustaining discharges, with light emission. Consequently, it may be assumed that the addressing stages contain at least one sustaining cycle.
To adjust the overall luminosity of an alternating type plasma panel, there is a known way of causing variation in the frequency of the sustaining signals. By making this frequency adjustable, the overall luminance of the panel is adjusted.
There is also a known way, described in the patent FR-2 662 292, of separating the selective (recording) operation from the semi-selective operation by an adjustable period that is substantially equal to a fraction of an image frame period, this fraction representing a percentage of the maximum luminosity. It may be recalled that the image frame period corresponds to the time needed to display an image.
It is increasingly being sought to display images in half-tones. In this type of display panel, each cell has several levels of illumination. The French patent FR-2 536 565 has proposed the processing, of all the rows of the panel several times and non-periodically in order to have several illumination periods for each cell.
This method uses several scans that are interleaved.
This method cannot be used to adopt the method for adjusting the overall luminosity which consists in separating the selective recording operation from the semi-selective operation since it is already necessary to distribute several commands for the recording and erasure of a row during a frame period.
Furthermore, it is difficult to adapt the method for adjusting the overall luminosity by variation of the sustaining frequency to the systems of half-tone displays using the above method since the processing rates of each row are imposed.
Up to now, no half-tone display method has been proposed enabling an adjustment of the overall luminosity.